Martin Luther's major contribution to theology was the idea of justification through faith alone. The Catholic Church's theology centered much more around the idea of justification (earning salvation) through works. It held that a person could earn salvation by participating in the proper church rituals, by doing good things, and even by paying money for indulgences. Luther believed that this could not be true. He argued that human beings were too sinful to ever be able to earn salvation through their own actions. They could only earn salvation by having faith in God.

This was not an idea that was completely new with Luther. However, he put it forward forcefully and at a time when it could really take root. It was the power of that idea that really drove the Protestant Reformation.